Page 140 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer
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Be st Practice 3 .3           Compressor Best Practices
          Every turbo-compressor must have its rotor system’s critical  - Maximum speed 6,600 rpm
       speeds determined prior to manufacture. In this section, we will  - Minimum speed 4,000 rpm
       follow the procedure for the determination of the necessary
                                                              Remember, changing of any value of support stiffness will
       parameters to define a rotor system’s critical speed. The pro-
       cedure is commonly known as determination of rotor response.  change the critical speed. Support stiffness in lbs/inch is plotted
       Figure 3.3.7 is a representation of a critical speed map for a rotor  on the x axis. The primary components of support stiffness, in
       system.                                              order of decreasing increasing influence are:
          It should be understood that all stiffness values are ‘calcu-  - Oil support stiffness
       lated’ and will vary under conditions of actual use. As an exer-  - Bearing pad or shell
       cise, determine NC 1 ,NC 2 and NC 3 for the horizontal and  - Bearing housing
       vertical directions for each bearing in Figure 3.3.7 (assume  - Bearing bracket
       bearing 1 and 2 stiffness are the same).             - Casing support foot
                                                            - Baseplate
        Critical speed      Horizontal (X)      Vertical (Y)  - Foundation

        NC 1                3,300 rpm           3,000 rpm     Note that this analysis of the critical speed does not include
                                                            oil film damping. It is common practice to first determine the
        NC 2                9,700 rpm           8,000 rpm
        NC 3                16,000 rpm          15,000 rpm
                                                            ‘undamped critical speeds’ to allow for necessary modifications
                                                            to the rotor or support system. This is because the effects of
          Based on a separation margin of   20% from a critical speed,  stiffness on the location of critical speed are significantly greater
       what would be the maximum allowable speed range between  than damping. Figure 3.3.7 shows four (4) distinct critical
       NC 1 and NC 2 in Figure 3.3.7?                       speeds. Operation within   20% of actual critical speeds is to be




















































       Fig 3.3.7   Compressor rotor critical speed map e no damping (Courtesy of Elliott Company)

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